Siamese bolt holes in powder metal components

ABSTRACT

A main bearing cap ( 20 ) has an hourglass shaped slot ( 22 ) in each leg ( 14 ).

CROSS-REFERENCE TO RELATED APPLICATIONS

This claims the benefit of U.S. Provisional Patent Application No.60/174,218 filed Jan. 3, 2000.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to powder metal component parts, and inparticular to a powder metal main bearing cap that reduces the totalweight of an internal combustion engine or similar device.

2. Discussion of the Prior Art

There is a continual pressure to reduce vehicular weight in order toimprove fuel efficiency and to conserve the world's finite oilresources. Major advances have been made in this direction, includingchanging the engine cylinder block from cast iron to aluminum. Attentionhas turned to all the smaller incremental weight savings that arepossible on every single component used in the engine.

It is necessary to retain an iron based (steel or cast iron) materialfor the main bearing cap that retains the crankshaft in its position atthe lower end of the cylinder block. The iron base is needed since ithas a matching thermal expansion rate with the crankshaft material,which is commonly either ductile cast iron or forged steel. In this way,as the engine temperature rises in service, the two components (bearingcap and crankshaft journal diameter) expand at the same rate, therebymaintaining constant running clearances at the bearing surfaces.However, since iron or steel is much more dense (about 300%) thanaluminum, weight reduction of the iron based bearing cap is a primetarget for weight reduction, especially since there are commonly 4 or 5caps per engine.

It is known to make main bearing caps by the manufacturing process knownas powder metallurgy. See, for example, International Patent PublicationNo. WO 97/42424. This process involves compaction of fine metallic andnon-metallic powder blends in precision tooling that generate a precisepowder preform or compact. This compact has low strength at this stage,only sufficient for handling without breaking as it is loaded onto acarrier plate that transports it through a thermal treatment furnace.This thermal treatment is known as “sintering”, and causes the powderparticles to inter-diffuse and bond together to form a strong “sintered”component that has many functional engineering uses. There are manypublications that explain the powder metallurgy (P/M) process in depthand practitioners of the art are knowledgeable in this process.

Although main bearing caps come in many different shapes, sizes andgeometries, a main bearing cap 2 is typified by the design shown in FIG.1. This consists of an essentially rectangular slab standing upon itslong surface, with a half-circular section cut out from the lower sideto create semi-circular surface 4 and legs 14 on each side of thesurface 4. This form then has the general appearance of a “bridge”. Theunderbridge semi-circular surface 4 forms the lower half of a round borewhen it is inverted and bolted to a mating semi-circular cut-out in thecylinder block. When the bearing caps are bolted to the cylinder block,they form the round-bore that holds the bearing shells that in turn holdthe crankshaft in place. During the power stroke of the engine, a greatload is placed upon the bearing caps that tries to separate the bearingcaps from the cylinder block. The resistance to separation is providedby the main bearing bolts that clamp the cap to the cylinder block. Asengine power has increased and cylinder block stiffness has decreased(aluminum is less stiff than cast iron), it has become the norm to usefour main bolts, two through holes 6, 8 on each side of the bore whichextend all of the way through the legs 14. Two more bolts are commonlyprovided at the outside ends of the cap, into blind tapped holes 10formed in wings 15 (hole 10 in right wing 15 is present but not shown),to further stiffen the assembly (see FIG. 1).

SUMMARY OF THE INVENTION

The invention provides a design feature in a sintered powder metal mainbearing cap, such as those used in combustion engines or similarreciprocating mechanical devices, which features a weight savingspecially shaped slot between adjacent bolt holes on each side of thebearing arch. The connecting slot may extend the full length of the boltholes, or for only the lower portion of the height, leaving a partialbridge to optimize under-bolt head clamping pressure. In addition, theouter profile of the bearing cap that surrounds the connected holesmimics the “hourglass” shape so as to keep a uniform wall thicknessaround the slot region. The precise shape and size of the slot iscalculated to ensure that the two bolts are guided into their respectivetapped holes by the narrowing of the slot in an “hourglass” shape thatalso maximizes the weight reduction from the metal removed by the slotform. The design and use of an hourglass shape section core-rod to formthe connected holes is also an advantage during powder compaction.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view partially in section of a typical prior artmain bearing cap;

FIG. 2 is a perspective view like FIG. 1 of a main bearing capincorporating the invention;

FIG. 3 is a perspective view like FIG. 2 of a second embodiment of amain bearing cap of the invention;

FIG. 4 is a fragmentary plan view from the plane of the line 4—4 of FIG.3;

FIG. 5 is a view like FIG. 4, illustrating a strain gage on theunderside of the slot bridge; and

FIG. 6 is a side view of the bearing cap of FIGS. 3-5, fitted with astrain gage on the underside of the slot bridge at each end andinstalled on a test fixture.

DETAILED DESCRIPTION OF THE INVENTION

This invention teaches a main bearing cap design that addresses theseconcerns, but in addition removes material from between the two sets ofmain bearing bolts. The bearing cap is made from aniron-copper-phosphorus alloy steel manufactured by powder metallurgy.

In the first embodiment of the invention illustrated in FIG. 2, mainbearing cap 20 has two bolt holes 16, 18 at each end which are co-joinedby a connecting slot 22 between them (FIG. 2), to make an hourglassshaped hole that accommodate both bolts, one at each end. The shape ofthe slot is important to the installation stage of engine build, whichis usually automated. Since the two bolts must fit into two respectivetapped holes in the cylinder block, the bolt holes must guide them intothese holes. A full width slot would not satisfy this function as thebolts could move towards each other and miss their respective holes.Therefore, the overall slot 28 must be shaped like an hourglass, withthe ends 16, 18 round and slightly larger in diameter than the bolts andthe connecting slot 22 smaller in width than the bolt diameter where theslot joins the holes, to ensure that the bolts still find theirrespective threaded holes during installation.

Since bolt sizes and hole clearances vary from engine to engine, asingle design can not be specified. However, the limiting “hourglass”geometry of the slot to maximize weight reduction yet provide boltguidance is an exercise in three-dimensional geometry that persons ofordinary skill in the art can readily accomplish.

A concern with a slot 22 which extends for the full length of the legs14 is that it reduces the clamping area under the bolt heads. This mayproduce excessive localized stresses in the region around the slot thatis in contact with the bolt head. The bolt head clamping loads can total20 tons on a typical vehicular bearing cap. To avoid an excessivepressure which may be caused by a full length slot 22, FIG. 3 shows asecond embodiment 32 in which slot 22 stops short of the top of thebearing cap 32 and a partial bridge 30 is left in place when thebetween-bolt slot 22 is formed. This small residual bridge 22 satisfiesthe under bolt head pressure concern and only marginally reduces theweight savings.

An initial test was carried out to confirm that the design of theinvention is sound. This test involved attaching a strain gage 40 to theunderside of the slot bridge in a powder metal main bearing cap asillustrated in FIG. 5. Next the cap was fitted and bolted into a testfixture that simulates the engine block using bolts 42, 44 and torqueprocedures specified by the engine manufacturer as illustrated in FIG.6. The leads of the gage were passed through a channel cut into the testblock. The maximum strain caused by this fitting and bolting wasmeasured and converted to a stress value using well known calculationsthat involve the bearing cap material elastic modulus, which in thisexample is 17.5 million psi. The resulting stresses from this test arebetween three and five thousand psi. This clearly indicates that thedesign is sound, since the stress is well below both the static strength(70 ksi) and the fatigue strength (26 ksi) of the chosen powder metalmaterial which was a copper phosphorus steel at a density of 6.5 to 6.7g/cc.

In the embodiment 32 of FIG. 3, the savings of weight of the enginetotaled 300 grams or 0.66 lbs. weight. This is a significant value,especially since when it is coupled with the “hourglass” outer profile50, best shown in FIGS. 4 and 5, that mimics the siamese shaped slot tomaintain a uniform wall thickness. As shown in FIGS. 4 and 5, the outerwidth of the profile 50 thins down in the length between the holes 16,18, so as to maintain a relatively constant wall thickness for the legs14. Thus, in the embodiment of FIGS. 3-5, the weight reduction isincreased to 590 grams (1.30 lbs.), a 9.6% weight savings.

Powder metallurgy is particularly well suited to making main bearingcaps with the relatively complex slot shapes as illustrated in FIG. 2 orFIGS. 3-6, since these shapes are formed with a core rod in the shape ofthe slot. An added benefit of the connected bolt-holes design relates tothese core rods. In a separate holes design (FIG. 1), the core rods aretwo rods to form the individual holes. These core-rods are thin andlong, and suffer from a tendency to bend and break under the highpressure used to compact the powders. The joining together of thebolt-holes results in a much stronger core rod and a reduced tendency tobend and break.

Preferred embodiments of the invention have been described inconsiderable detail. Many modifications and variations will be apparentto persons of ordinary skill in the art. Therefore, the invention shouldnot be limited to the embodiments described.

We claim:
 1. In a main bearing cap which has a half bore formed in itand a pair of legs, each said leg being on opposite sides of thehalf-bore and each said leg being boltable by two bolts to a supportingstructure so as to secure said bearing cap to said supporting structureso that said half-bore of said bearing cap mates with a half bore insaid supporting structure to form a fill bore, the improvement whereinsaid main bearing cap is powder metal and each said leg has formed in itan hourglass shaped slot having opposite ends enlarged relative to anopen central section which connects said two ends, each said end beingsized to receive one of said two bolts and said central section beingsized so as to be smaller in width than said end sections so that saidend sections guide said bolts as they are inserted through said legs. 2.The improvement of claim 1, wherein said hourglass shaped slot endsbelow an upper surface of said main bearing cap, and through holes areprovided in said upper surface which are aligned with the ends of saidslot so as to provide a bridge of material between said holes underheads of said bolts.
 3. The improvement of claim 1, wherein the exteriorsides of each leg are also hourglass shaped to provide an approximatelyuniform wall thickness between at least portions of said exterior sidesand said hourglass shaped slot.
 4. The improvement of claim 1, whereineach said slot is formed by a single core rod during compaction of saidmain bearing cap.